Raw natural gas for liquefaction is generally relatively clean. Where the raw gas contains contaminants such as water, carbon dioxide and hydrogen sulfide, the gas generally is treated prior to liquefaction to remove these contaminants. As more of the total worldwide gas production is liquefied for ease of handling and transport, the raw natural gas streams more frequently contain excessive mercaptan levels.
Conventionally, the mercaptans have been removed by pretreating the natural gas feed stream with either a physical or chemical solvent, or a molecular sieve. Where high levels of mercaptans are encountered, removal techniques specific to mercaptans must be used in addition to the treatment process for carbon dioxide and hydrogen sulfide.
The use of the physical or chemical solvent systems is expensive and complicated from an operational standpoint. The use of molecular sieves, on the other hand, requires very large beds and bed regeneration can require a volume of regeneration gas as much as 80 percent of the gas feed stream. Also, the off gas produced by regeneration has a very high mercaptan concentration. Such high mercaptan concentrations in the regeneration off gas are typically too severe to be tolerated by a fuel gas system. Also, the regeneration of large molecular sieves results in a substantial fluctuation of the propane, butane and heavier hydrocarbons introduced into a fuel gas system.
Therefore, a need exists for a method for liquefying raw, mercaptan-containing natural gas which avoids the problems and disadvantages associated with the aforementioned prior art natural gas pretreatment methodologies.